This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to an h-infinity controller which provides control inputs to a disc drive actuator to maintain a head adjacent a selected track on a rotatable disc.
Disc drives are used as primary data storage devices in modern computer systems and networks. A typical disc drive comprises a head-disc assembly (HDA) which houses mechanical portions of the drive, and a printed circuit board (PCB) mounted to an outer surface of the HDA which supports electronic circuitry used to control the HDA.
A typical HDA includes one or more rigid magnetic storage discs which are journaled about a rotary hub of a spindle motor to form a rotatable disc pack, and an actuator assembly which supports an array of heads adjacent tracks defined on the disc surfaces. An actuator motor, such as a voice coil motor (VCM), rotates the actuator assembly, and hence the heads, across the disc surfaces. The control circuitry on the PCB includes a read/write channel which interfaces with the heads to transfer data between the tracks and a host computer, and a servo control system which drives the VCM to provide head positional control.
Continued demand for disc drives with ever increasing levels of data storage capacity have led disc drive manufacturers to seek ways to increase the density of data bits that are stored on each disc surface. High performance disc drives of the present generation typically achieve area bit densities measured in several gigabits per square centimeter, Gbits/cm2. Higher recording densities can be achieved by increasing the number of bits that are stored along each track, and/or by increasing the number of tracks per unit width across each disc. Storing more bits along each track generally requires improvements in the read/write channel electronics to enable the data to be written (and subsequently read) at a correspondingly higher frequency. Providing higher track densities generally requires improvements in the servo control system to enable the heads to be more precisely positioned over the discs.
In an effort to achieve higher track densities, disc drive manufacturers are increasingly moving to implement so-called xe2x80x9cmicroactuators,xe2x80x9d which are secondary motors suspended by actuator assemblies to provide fine (secondary) positional adjustment of the heads over and above the coarse (primary) positional adjustment provided by the VCMs. A variety of microactuator constructions have been recently proposed in the art, including the use of an inductive rotor/stator arrangement as exemplified by U.S. Pat. No. 5,657,188 issued to Jurgenson et al.; the use of piezoelectric transducers as exemplified by U.S. Pat. No. 6,002,549 issued to Berman et al.; and microelectronic machine (MEM) implementations as discussed in U.S. Pat. No. 5,711,063 issued to Budde et al.
The basic operational concept of an actuator assembly with both primary and secondary motors (also referred to as a xe2x80x9cdual-stagexe2x80x9d actuator) is relatively straightforward; the primary motor is used to bring the selected head within a given range of tracks, after which the secondary motor operates to bring the head over one particular selected track from the range of tracks. Implementation of the servo control electronics to provide the necessary control signals to the primary and secondary motors, however, has thus far proved to be a rather daunting task.
At present, one proposed solution is to provide a high performance programmable device (such as a digital signal processor, or DSP) with extensive programming algorithms in order to carry out the required dual-stage servo control. Such an approach is discussed, for example, by United States Pat. No. 5,978,752 issued to Morris. While operable, there is a continued need for robust and cost-effective controller solutions that can be implemented for both high performance single stage and dual stage actuators in a cost-effective manner and which are easily adapted to different design configurations and to different head/media combinations.
The present invention provides an apparatus and method for improving disc drive actuator control.
In accordance with preferred embodiments, a disc drive is provided with a rotatable actuator which supports an array of read/write heads adjacent a corresponding number of recording surfaces in a rotatable disc stack. A servo engine, characterized as a high-speed hardware circuit, generates control inputs for the actuator to carry out track following operations wherein a selected head is caused to follow a corresponding track.
The actuator can be characterized as a single-stage actuator having a single actuator motor (such as a voice coil motor, VCM), or alternatively can be characterized as a dual-stage actuator having both a primary actuator motor (VCM) and an array of secondary microactuators which controllably move each head. The control inputs for the motor or motors are determined in relation to an actual position signal for the head, a desired position signal for the head and a unique set of coefficients, or table lookup values, that are individually determined and loaded for each head.
The coefficients are selected by obtaining an output response for each head, selecting the coefficients in relation to the output response for each head, and storing the coefficients in a nonvolatile memory location of the disc drive. The appropriate set of coefficients is thereafter loaded into the hardware controller as each head is selected in turn during normal drive operation.
Each set of coefficients comprises the results of intermediate calculations for state equations used to determine the control input values. In this way, the hardware controller efficiently obtains solutions for the control inputs for the primary and secondary motors in a number of clock cycles corresponding to the length of the multi-bit input (actual position and desired position) words, by serializing the input words and using these as addresses to access the appropriate coefficients. The hardware controller further advantageously allows track following operations to be offloaded from a programmable processor of the disc drive, which enables the disc drive design to utilize a single processor for top level control of data transfer operations and velocity controlled seek operations.
The use of different sets of coefficients by head enables the hardware controller to be adapted for each head/disc combination. That is, as each new head is selected, a new set of table lookup values appropriate for the new head are loaded into the hardware controller, providing accurate, adaptive control. Moreover, dual-state actuators have inherently complex response characteristics, and the use of h-infinity controller synthesis provides an efficient way to reduce the uncertainties associated with dual-stage actuator design.
The servo engine is advantageously incorporated into an application specific integrated circuit (ASIC) housing other related circuits of the disc drive, such as demodulation (demod) circuitry used to condition servo signals readback from the disc surfaces. A special purpose random access memory (RAM) configuration is used to significantly reduce the time required to load each set of coefficients as each new head is selected in turn.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.